Many more such studies can be found in databases such as PLos ONE,SSRN or Google Scholar. When searching, try to find studies that are published in research-oriented journals, that come from universities or non-partisan research organizations, and that cite other studies published in journals.

Here are some other places to look for more information in this area:

U.S. Government

Science.gov: Links to authoritative scientific documents, reports and position papers from a variety of U.S. Federal agencies.